The present invention relates to the evaluation of a subterranean formation penetrated by a wellbore. More particularly, the present invention relates to techniques for deriving at least one formation parameter from signals generated by a downhole tool positioned in the wellbore.
The exploration of hydrocarbons involves placement of a downhole tool into the wellbore to perform various downhole operations. There are many types of downhole tools used in downhole operations. Typically, a drilling tool is suspended from an oil-rig and advanced into the earth to form the wellbore. The drilling tool may be a measurement-while-drilling (MWD) or a logging-while-drilling (LWD) tool adapted to perform downhole operations, such as taking measurements, during the drilling process. Such measurements are generally taken by instruments mounted within drill collars above the drill bit and may obtain information, such as the position of the drill bit, the nature of the drilling process, oil/gas composition/quality, pressure, temperature and other downhole conditions.
In some instances, it may be desirable to obtain additional data from the wellbore after drilling is complete. In such cases, the downhole drilling tool may be provided with downhole evaluation systems adapted to collect downhole information. Alteratively, the downhole drilling tool may be removed, and a separate downhole evaluation tool, such as a wireline, slickline, drill stem test or coiled tubing tool, may be lowered into the wellbore to perform additional testing, sampling and/or measuring.
Downhole evaluation tools may be provided with communication systems adapted to send signals, such as commands, power and information, between a downhole unit housed in the downhole tool, and a surface unit. Communication systems in drilling tools may include, for example, mud pulse systems that manipulate the flow of drilling mud through a downhole drilling tool to create pressure pulses. One such mud pulse system is disclosed in U.S. Pat. No. 5,517,464 and assigned to the assignee of the present invention. Other communication systems, such as wired drill pipe, electromagnetic, acoustic or other telemetry systems may also be provided. Downhole wireline tools typically communicate through the armored wired cable used as the conveyor for the wireline tool.
In some instances, such as when the communication system is unavailable, inactive or detached, such as during memory mode logging, data is collected and stored in a memory unit within the downhole tool for later retrieval. By way of example, some wireline tools are deployed into the wellbore without the wireline connection between the surface system and the downhole tool. The use of a wireline can be too risky to use, or too costly to justify the expense. The wireline cable may be detached, and the logging tool operated using self-contained power supplies (usually batteries) and data memory units (data memory and circuitry to bus the data from the sensors). Such a tool is placed in operation at the surface, then lowered into the wellbore by a conveyor, or dropped or pumped down the wellbore. The tool may be moved past multiple depth intervals, or it may be left at a single depth in the well. Regardless, the tool will record well data and store the data in memory for collection by the operator at some future time, such as when the tool is returned to the surface. During this type of ‘memory mode’ logging, the operator typically has no communication with the tool to ensure that the tool is working properly throughout the operation, to turn the tool off and on, to change the type of data collected by the tool, or to change the frequency at which the data is collected. The data collected during memory mode logging is typically retrieved by establishing (or re-establishing) a wired or mud pulse communication link between the downhole tool and the surface, or by retrieving the tool to the surface and downloading the information from the memory unit.
Wireless communication techniques, such as electromagnetic (or emag) telemetry systems, having been employed in downhole drilling tools. Such systems include a downhole electromagnetic communication unit that creates an electromagnetic field capable of sending a signal to a remote surface electromagnetic communication unit. Examples of a downhole electromagnetic communication unit are disclosed in U.S. Pat. No. 5,642,051 and U.S. Pat. No. 5,396,232, both of which are assigned to the assignee of the present invention. Current downhole electromagnetic communication units have been used in conventional MWD type drilling operations.
Advancements, such as the use of repeaters and gaps, have been implemented in existing downhole tools to improve the operability of electromagnetic systems in downhole applications. The gap, or non-conductive insert, is positioned between adjoining sections of drill pipe to magnify the electromagnetic field and provide an improved signal. Examples of a gap used in a downhole electromagnetic communication unit are described in U.S. Pat. No. 5,396,232, assigned to the assignee of the present invention and U.S. Pat. No. 2,400,170 assigned to Silverman.
Communication systems are typically positioned in downhole tools and used to convey information collected by the downhole tool to a surface unit for analysis. Downhole tools are often used to perform formation evaluation to collect information about the subterranean formations. The downhole tools are provided with components capable of measuring formation parameters, such as pressure, temperature, permeability, porosity, density, viscosity, resistivity and more. This collected information is transferred to the surface using the communication systems.
Resistivity of the formation is one such formation parameter collected during downhole formation evaluation. Resistivity is an important parameter to understand and increase reservoir production. This is largely because of the rule that water conducts electricity and hydrocarbons do not. If the formation resistivity and its porosity are known; an estimate can be made of the fluid in the pore spaces. An example of a technique describing a resistivity measurement is provided in U.S. Pat. No. 6,188,222.
Despite the advancement in communication and formation evaluation, there remains a need to provide low cost and efficient alternatives to existing techniques. It is desirable that such techniques eliminate the need for duplicate devices and/or operations to perform telemetry and formation evaluation operations. It is further desirable that such techniques reduce the costs and complexities associated with the existing resistivity measurement and electromagnetic telemetry tools. It is, therefore, desirable to provide techniques that provide the ability to measure at least one parameter of the subterranean formation while passing electromagnetic signals through the formation using the electromagnetic telemetry system.